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The tandem pump delivers oil to the high speed load sensed unloader valve, where the two flows can be combined to give high speed operation, or split to give low speed with the second pump flow returning to the oil reservoir.
Also fitted to this valve is a solenoid operated dump valve that is activated by the joystick controllers and the emergency stop buttons.
The pump flow from the high speed load sensor (LS) unloader valve passes through the high pressure filter and is divided and supplied to the front and rear control valves. These valves are connected to each other with a load sense line. This helps to maintain synchronisation of the crane lifting arms when unequal loads are handled.
Fitted into the load sense line is an optional solenoid operated dump valve. When the valve is in the de-energised condition, load sensed (LS) oil is dumped to tank. When the valve is energised, the LS line is blocked to tank and allows the hydraulic system to operate.
A hydraulic gauge is fitted into the LS line and indicates the pressure in the system.
The crane control valves are Danfoss PVG 32 proportional type and are signalled from the joysticks for directional control. High or low speed can be selected at the remote control. The low speed function only applies to the lifting arms. The stabiliser legs are always in high speed irrespective of the position of the high/ low speed selector switch as they do not require the fine control required for the lifting arms.
On analogue units when "Low Speed" is selected only one pump is supplying oil to the system. The joystick signals pass via an electronic hydraulic flow (EHF) control and the function speed is reduced by 50% to allow fine control under heavy and difficult loading conditions.
Digital units use a different method to provide a 50% low speed operation.